Start by looking to the product there that doesn't have coefficient. Need to have exactly 2 Fe on the reactant side. So first coefficient is 2.. having a 2 there gives 2*3 = 6 H on the reactant side. Then in order to have 6 H on the product side the coefficient there needs to be 3. This gives 3 Ca on product side so there must be 3 Ca on reactant side.
coefficients:
2 + 3 --> 1 + 3
